This invention relates to phthalocyanine photoconductive materials and, more particularly, to a novel process for the preparation of X-form metal-free phthalocyanine.
It is known that images may be formed and developed on the surface of certain photoconductive materials by electrostatic means. The basic xerographic process, as taught by Carlson in U.S. Pat. No. 2,297,691, involves uniformly charging a photoconductive insulating layer and then exposing the layer to a light-and-shadow image which dissipates charge on the portions of the layer which are exposed to light. The electrostatic image formed on the layer corresponds to the configuration of the light-and-shadow image. This image is rendered visible by depositing on the imaged layer a finely divided developing material comprising a colorant called a toner and a toner carrier. The powder developing carrier will normally be attracted to those portions of the layer which retain a charge, thereby forming a powder image corresponding to the latent electrostatic image. This powder image may then be transferred to paper and other receiving surfaces. The paper then will bear the powder image which may subsequently be made permanent by heating or other suitable fixing means. The above general process is also described in U.S. Pat. Nos. 2,357,809; 2,891,011, and 3,079,342.
It has recently been found that an especially sensitive electrophotographic plate may be prepared by mixing metal-free phthalocyanine in a binder, coating the mixture onto a conductive substrate and hardening the binder. Electrophotographic plates comprising phthalocyanines in a binder are described in detail in copending application Ser. No. 518,450, now abandoned, filed Jan. 3, 1966. It was found that an especially sensitive form of metal-free phthalocyanine could be prepared by extended dry milling or grinding of alpha or beta form metal-free phthalocyanine. This highly sensitive material was found to be a new polymorphic form of phthalocyanine. This new polymorph, now referred to as "X-form" phthalocyanine is described in detail and claimed in U.S. Pat. No. 3,357,989. While the X-form phthalocyanine produced by grinding or milling has excellent physical properties, this method of preparation has several disadvantages. In order to arrive at a complete conversion of alpha or beta metal-free phthalocyanine to the corresponding X-form phthalocyanine often requires very extended periods of milling, often over 100 hours. Besides being time consuming, this process consumes a large amount of power and requires large and sometimes complex milling equipment. Moreover, impurities may be introduced into the phthalocyanine dispersion from the metallic or ceramic milling equipment. It is very difficult to predict the milling time or total work required in a particular milling machine to insure complete conversion to X-form phthalocyanine. When changes are made in milling equipment or techniques, test runs must be made with frequent sampling and X-ray or infrared examination to detect when complete conversion has been attained.
In copending application Ser. No. 566,839, now abandoned a second method for the preparation of X-form phthalocyanine is disclosed and claimed. This method comprises mixing the alpha crystalline form of metal-free phthalocyanine with a portion of the X-form and an aliphatic organic solvent, and maintaining the mixture until the alpha form is converted to the X-form. While the X-form phthalocyanine produced by this method, like the X-form material produced by grinding or milling, has excellent physical properties, there is also several disadvantages associated with said method. For example, in order to obtain complete conversion of alpha to X-form a time period of at least 16 hours is required. Aside from being very time consuming, this process proves expensive on a large scale production of X-form phthalocyanine, wherein great amounts of aliphatic organic solvents are consumed. Further, one must employ alpha metal-free phthalocyanine, rather than the corresponding beta form, as a starting component, in order to arrive at the resulting X-form. While it is true that beta metal-free phthalocyanine may be converted to alpha metal-free polymorph by dissolving it in 98% sulfuric acid solution and precipitating the solution in ice water, this conversion technique proves both difficult and expensive. Sulfuric acid tends to degrade phthalocyanine resulting in the formation of phthalimide, phthalic acid, and various nitrogen containing compounds which are intolerable in an electrophotographic system. Moreover, since it is difficult to extract sulfuric acid from the reprecipitated phthalocyanine, it is probable that there is a continuous degradation of the phthalocyanine due to residual acid.
Copending application Ser. No. 755,411, now abandoned filed in the U.S. Patent Office Aug. 30, 1968, discloses a method of preparing X-form metal-free phthalocyanine which comprises the steps of mixing phthalonitrile in an ammonia-saturated alkylalkanolamine solvent, seeding the mixture with a catalytic amount of X-form phthalocyanine, heating said mixture to reflux temperature and maintaining said temperature for about 20 to about 70 minutes, and filtering the hot reaction product formed thereby. While this particular method is faster in time than the two previously discussed methods, it has the disadvantage of requiring several steps and relatively costly solvents and other starting materials. Copending application, Ser. No. 756,362, now abandoned filed in the U.S. Patent Office Aug. 30, 1968, discloses and claims still another method of preparing X-form metal-free phthalocyanine which comprises sublimating any metal-free phthalocyanine polymorph under a pressure of about 10.sup.-1 to about 10.sup.-6 Torr. This method, while quite fast and while producing a high yield of substantially pure X-form metal-free phthalocyanine, requires the use of complex and expensive evaporation equipment, especially when it is desired to industrially prepare X-form metal-free phthalocyanine on large scale.
Since large quantities of X-form metal-free phthalocyanine of high purity may be needed for electrophotographic or other uses, there is a continuing need for a simpler, cheaper, less time consuming, and more reproducible method of preparing same.
It is, therefore, an object of this invention to provide a method of preparing X-form metal-free phthalocyanine and phthalocyanine compounds devoid of the above-noted disadvantages.
Another object of this invention is to provide a one-step direct method for the preparation of X-form metal-free phthalocyanine and phthalocyanine compounds.
Still another object of this invention is to provide a method of preparing substantially pure X-form metal-free phthalocyanine and phthalocyanine compounds wherein the yield of the resulting product is very high.
It is yet another object of this invention to provide a simple and extremely rapid method of preparing X-form metal-free phthalocyanine and phthalocyanine compounds.
A further object of this invention is to provide a reliable and dependable method of preparing X-form metal-free phthalocyanine and phthalocyanine compounds which gives highly reproducible results.
Yet another object of this invention is to provide a method of preparing X-form metal-free phthalocyanine and phthalocyanine compounds which requires inexpensive heating and cooling techniques.
Another further object of this invention is to provide an economical method of preparing X-form metal-free phthalocyanine and when it is to be produced on a large scale industrial level.